Training problem with deep neural network model
Without seeing your code and/or data, it's going to be difficult to give a concrete answer.

Possibly reasons might include:


Your model isn't complex enough to fit the patterns in the data, and so it reaches a local minimum in the error function.
The patterns in the data are not self-consistent, and so this is actually the global minimum in the error function.
There's something wrong with your learning rate parameter, or with the gradient in the problem. Try increasing the learning rate, and ensure your model is being initialized with random weights.